1. Field of the Invention This invention relates to a hybrid constant current LED lamp and, more particularly, to a constant current LED lamp having a hybrid driver circuit capable of providing a constant current with a minimal power loss.
2. Description of the Related Art
As the conventional lamp equipments have long suffered from the drawbacks of high power consumption and short service life, LED (light-emitting diodes) for general illumination applications are gaining popularity for their lower power consumption and longer service life.
However, since the LED have to be provided with a driver circuit to convert the AC power of the mains to DC, as the LED are normally driven by DC current, any improvement of the efficiency of the driver circuit would have far reaching effects.
FIG. 1 shows a conventional LED driver circuit, in which a resistor R is connected in series to LED 12. The resistor R is adjusted to consume excess voltage from the power source, so that the LED 12 would receive power with a suitable input current. However, this driver circuit has been shown to exhibit poor conversion efficiency.
FIG. 2 shows another conventional LED driver circuit 1 connected to at least one LED 12 and adapted to receive an AC power VAC. The LED driver circuit 1 is a switching mode power supply, and mainly comprises of a bridge rectifier 10, an electromagnetic interference restraining circuit 11, a filter circuit 16, a high frequency switching circuit 13, a converter circuit 14, and a feedback circuit 15, wherein the bridge rectifier 10 is used to rectify the VAC into a pulsed DC power, while the filter circuit 16 dampens the voltage swings of the DC power. The frequency of the DC power is controlled by the high frequency switching circuit 13, and the converter circuit 14 is used to convert the high voltage into a low voltage DC power that is supplied to the LED 12. The feedback circuit 15 can detect the magnitude of total voltage load of the LED 12, thereby regulates the output voltage of the converter circuit 14.
Although the conventional LED driver circuit 1 does drive the LED 12 to emit light, the architecture of the driver circuit 1 is quite complicated, resulting in a large circuit and high cost. Moreover, the voltage conversion circuit 14 is positioned close to the power input terminal and, as a consequence, a significant amount of electricity tends to lose in the form of heat during voltage conversion, causing a temperature rise in the circuit. This adds up to a lot of wasted energy and reduces the service life of the LED products.
U.S. patent application Ser. No. 13/080,850 discloses another LED driver circuit used to drive multiple LED, or LED packages 12 that comprise multiple chips, connected in series. As shown in FIG. 3, the driver circuit comprises a rectifier circuit 17, a filter circuit 16, a stable voltage circuit 18 and a constant current circuit 19. The rectifier circuit 17 is adapted to receive an alternating current power and converting the alternating current power into a direct current power. The filter circuit 16 is connected to the rectifier circuit 17 and transmits the DC power to the stable voltage circuit 18 and the LED 12, respectively. The stable voltage circuit 18 is connected to the rectifier circuit 17 and comprises a first resistor 181 and a voltage stabilizer 182 (which may by way of example be a Zener diode) connected in series for generating a constant voltage. The constant current circuit 19 is connected to the stable voltage circuit 18, and comprises a transistor 191 connected in series to the LED 12 to limit the driving current flowing into the LED 12 and to maintain the current at a constant value.
Although the conventional LED driver circuit described above is capable of supplying a constant driving current to the LED 12, the transistor 191 is installed to absorb excess voltage from the input AC power. As a result, the excess voltage is absorbed by the transistor 191 and dissipated as waste heat.